The invention relates to a circulating fluidized bed device provided with an oxygen-fired furnace (1), a separator (2) for separating gas and solid particles, a return circuit for returning solids to the furnace (3), and an outlet duct (4) leaving the separator and conveying gases to a heat-recovery boiler, the device being characterized in that it includes injector means for injecting combustible) gas into the outlet from the separator 2), said combustible gas being propelled by recycled carbon dioxide into the residual vortex created at the outlet from the separator, so as to ensure intimate mixing takes place in the downstream duct (4) with the combustion flue gases, thereby reducing the oxygen content of said flue gases by residual combustion.